feat: distro/DE selector UI and real, verified proot invocation flags

app/src/main/assets/de_packages.yaml

@@ -40,6 +40,10 @@ des:
   - alias: xfce4
     display_name: "XFCE4"
     tier: 1
+    # Run inside the container's already-logged-in boot_command session
+    # (see ProotExec.bootCommandTemplate()) once the X11 socket is ready —
+    # NOT a separate proot invocation.
+    session_command: "dbus-launch --exit-with-session startxfce4"
     package_managers:
       pacman: ["xfce4", "xfce4-goodies", "tigervnc", "dbus"]
       dnf: ["@xfce-desktop-environment", "tigervnc-server"]
@@ -48,6 +52,7 @@ des:
   - alias: lxqt
     display_name: "LXQt"
     tier: 1
+    session_command: "dbus-launch --exit-with-session startlxqt"
     package_managers:
       pacman: ["lxqt", "tigervnc", "dbus"]
       dnf: ["@lxqt-desktop-environment", "tigervnc-server"]
@@ -56,6 +61,7 @@ des:
   - alias: mate
     display_name: "MATE"
     tier: 1
+    session_command: "dbus-launch --exit-with-session mate-session"
     package_managers:
       pacman: ["mate", "tigervnc", "dbus"]
       dnf: ["@mate-desktop-environment", "tigervnc-server"]
@@ -64,6 +70,7 @@ des:
   - alias: i3wm
     display_name: "i3wm"
     tier: 1
+    session_command: "dbus-launch --exit-with-session i3"
     package_managers:
       pacman: ["i3-wm", "i3status", "dmenu", "tigervnc", "dbus"]
       dnf: ["@i3-desktop-environment", "tigervnc-server"]

app/src/main/java/com/fct/tc4/ui/main/MainActivity.kt

@@ -42,6 +42,8 @@ import com.fct.tc4.ui.misc.WipeLayout
 import com.fct.tc4.ui.page.ContainerInstallFragment
 import com.fct.tc4.ui.page.ContainerMainFragment
 import com.fct.tc4.ui.page.ContainerManageFragment
+import com.fct.tc4.ui.page.DeSelectFragment
+import com.fct.tc4.ui.page.DistroSelectFragment
 import com.fct.tc4.ui.misc.Global
 import com.fct.tc4.ui.page.TerminalFragment
 import com.fct.tc4.ui.page.ContainerManageViewModel
@@ -165,6 +167,10 @@ class MainActivity : AppCompatActivity() {
                 fragment?.onImportAction()
                 true
             }
+            R.id.containerBuildNew -> {
+                viewModel.navigateTo(MainViewModel.Screen.DistroSelect)
+                true
+            }
             R.id.enterGui -> {
                 val fragment = supportFragmentManager.findFragmentByTag("ContainerMain") as? ContainerMainFragment
                 fragment?.onEnterGui()
@@ -215,6 +221,14 @@ class MainActivity : AppCompatActivity() {
                 }
                 tag = "ContainerMain"
             }
+            is MainViewModel.Screen.DistroSelect -> {
+                fragment = DistroSelectFragment()
+                tag = "DistroSelect"
+            }
+            is MainViewModel.Screen.DeSelect -> {
+                fragment = DeSelectFragment.newInstance(screen.distroAlias)
+                tag = "DeSelect"
+            }
         }
         if (supportFragmentManager.findFragmentByTag(tag) != null) return
         supportFragmentManager.commit {

app/src/main/java/com/fct/tc4/ui/main/MainViewModel.kt

@@ -40,6 +40,10 @@ class MainViewModel(application: Application) : AndroidViewModel(application) {
         data object ContainerManage : Screen()
         data class ContainerInstall(val code: String, val webpage: String?) : Screen()
         data class ContainerMain(val code: String) : Screen()
+        /** First step of the on-device distro/DE build flow — pick a base distro. */
+        data object DistroSelect : Screen()
+        /** Second step — pick a desktop environment for the [distroAlias] chosen in DistroSelect. */
+        data class DeSelect(val distroAlias: String) : Screen()
     }
 
 

app/src/main/java/com/fct/tc4/ui/misc/DeRegistry.kt

@@ -30,6 +30,15 @@ data class DeEntry(
     val requiresBridge: Boolean,
     /** package_manager family name -> list of packages/groups to install. */
     val packageManagers: Map<PackageManagerFamily, List<String>>,
+    /**
+     * Command run inside the container's already-logged-in boot_command
+     * session once the X11 socket is ready (see TinyYamlBuilder.kt) — null
+     * for every current tier-2 entry, since how a Wayland-only compositor
+     * actually gets a picture onto Termux:X11's X-server-shaped bridge is
+     * exactly the unprototyped question the task brief asked to flag rather
+     * than guess at.
+     */
+    val sessionCommand: String?,
 ) {
     /** Null if this DE has no known install command for [family] yet (see de_packages.yaml). */
     fun packagesFor(family: PackageManagerFamily): List<String>? = packageManagers[family]
@@ -83,7 +92,8 @@ object DeRegistry {
         val packageManagers = packageManagersRaw.entries.associate { (key, value) ->
             PackageManagerFamily.fromString(key) to value
         }
+        val sessionCommand = raw["session_command"] as? String
 
-        return DeEntry(alias, displayName, tier, requiresBridge, packageManagers)
+        return DeEntry(alias, displayName, tier, requiresBridge, packageManagers, sessionCommand)
     }
 }

app/src/main/java/com/fct/tc4/ui/misc/TinyYamlBuilder.kt

@@ -0,0 +1,77 @@
+// TinyYamlBuilder.kt -- This file is part of tiny_container.
+//
+// Copyright (C) 2026 Caten Hu
+//
+// Tiny Container is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published
+// by the Free Software Foundation, either version 3 of the License,
+// or any later version.
+//
+// Tiny Container is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty
+// of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+// See the GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see http://www.gnu.org/licenses/.
+
+package com.fct.tc4.ui.misc
+
+/**
+ * Synthesizes the config map a freshly on-device-built (distro + DE)
+ * container needs — every other container's config comes from an
+ * upstream-authored .tiny.yaml imported wholesale; this is the one built by
+ * this app itself rather than imported.
+ */
+object TinyYamlBuilder {
+
+    /** Matches the DISPLAY=:6 baked into ProotExec.bootCommandTemplate(). */
+    private const val X11_DISPLAY = "6"
+
+    /**
+     * @throws IllegalStateException if [de] is non-null but has no
+     *   [DeEntry.sessionCommand] (every current tier-2 entry) — there is
+     *   nothing correct to write into `feature[].command` in that case, so
+     *   this refuses rather than shipping a container with a guessed,
+     *   probably-broken session launch command. Callers should check
+     *   `de.sessionCommand != null` before offering a tier-2 DE as buildable.
+     */
+    fun build(code: String, distro: DistroEntry, de: DeEntry?): Map<String, Any> {
+        val sessionCommand = de?.let {
+            it.sessionCommand
+                ?: throw IllegalStateException(
+                    "DE '${it.alias}' has no session_command yet — its X11/Wayland launch " +
+                        "path isn't validated, so there's nothing correct to write here.",
+                )
+        }
+
+        val name = "${distro.displayName}${de?.let { " (${it.displayName})" } ?: ""}"
+        val description = buildString {
+            append("Built on-device from ${distro.displayName}")
+            if (de != null) append(" with ${de.displayName}")
+            append(".")
+        }
+
+        val config = mutableMapOf<String, Any>(
+            "code" to code,
+            "name" to name,
+            "description" to description,
+            "boot_command" to ProotExec.bootCommandTemplate(),
+        )
+
+        if (sessionCommand != null) {
+            config["feature"] = listOf(
+                mapOf(
+                    "type" to "x11",
+                    "name" to "X11",
+                    "description" to "Termux:X11 graphical session.",
+                    "enabled" to true,
+                    "args" to listOf(":$X11_DISPLAY", "-extension", "MIT-SHM"),
+                    "command" to sessionCommand,
+                ),
+            )
+        }
+
+        return config
+    }
+}